Demands for on-vehicle camera apparatuses for use in blind corner monitors and rear-view monitors on vehicles have increased in recent years. An image-capturing environment for such an on-vehicle camera apparatus is not always bright; i.e., the image-capturing environment is sometimes relatively dark. Because the brightness of an image frame captured in a relatively dark image-capturing environment is insufficient, against which some countermeasure is required.
In a camera apparatuses that includes an imaging device such as a CCD, when the brightness of the image frame is insufficient, it is typical to adopt a scheme of increasing the gain of an automatic gain control circuit (AGC) of the imaging device thereby obtaining a brighter image. Such a technology has been disclosed, for example, in Japanese Patent Application Laid-open No. H4-247777. Meanwhile, increase in the gain disadvantageously leads to increasing in the noise. As a countermeasure thereagainst, conventionally a scheme of enabling a low-pass filter in a dark portion of the image frame where the brightness is not sufficient thereby reducing noise has been generally employed. A low-pass filter has also been generally implemented by using an infinite impulse response (IIR) filter in circuit of relatively small scale. Such a technology has been disclosed, for example, in Japanese Patent Application Laid-open No. 2005-184786.
Accordingly, use of an IIR filter on an on-vehicle camera apparatus or the like leads to the possibility of increasing the gain in the dark portion to maintain the brightness of the image to a desirable brightness without increasing the noise.
An on-vehicle camera apparatuses or the like typically uses a wide-view-angle optical system and includes, in its image processing unit, a high-frequency enhancement filter, such as a finite impulse response (FIR) filter, for the purpose of correcting degradation in resolution resulting from the optical system. However, a line buffer with a capacity of several lines is indispensable for this high-frequency enhancement filter. Meanwhile, the scheme of implementing a low-pass filter to suppress noise by using an IIR filter requires an additional line buffer. However, under recent circumstances where capacity necessary for a line buffer has been greatly increased due to increase in the number of pixels in imaging devices, providing an additional line buffer for an IIR filter results in increase in circuit scale.
The present invention aims at providing an image processing apparatus that enhances high-frequency components of an image that are attenuated by an optical system and reduces noise in a dark portion without increasing circuit scale and that is of low cost and low power consumption, and an on-vehicle camera apparatus that includes the image processing apparatus.